The presence and neuroanatomical location of separate visual pathways for spatial vision and object vision were investigated in healthy young men by measuring regional cerebral blood flow (rCBF) with positron emission tomography (PET) and H215O. The results identified extrastriate visual areas associated with these pathways: dorsolateral occipital and superior parietal areas activated more by spatial visual processing, and ventral occipital and occipitotemporal areas activated more by object discrimination. Old subjects demonstrated rCBF activations in the same regions as did young subjects, but also demonstrated activation of ventral areas during spatial vision and dorsal areas during object vision, suggesting less functional separation of these visual systems. Patients with dementia of the Alzheimer type (DAT) demonstrated normal percent baseline activation of rCBF in occipitotemporal cortex during object vision, suggesting preserved capacity to recruit this area for perceptual processing. In healthy adults, significant age-related differences were found on tests of perceptual reasoning and visual memory, but not on verbal memory and verbal intelligence. Age-related visual memory differences could be accounted for by the age-related differences in visual cognitive processing, suggesting that they are restricted to visuospatial performance with relative preservation of memory functions. Neuropsychological decline in very early Alzheimer's disease was found to have an early plateau phase followed by steady, linear decline. Rate of cognitive decline is significantly correlated with rate of brain tissue loss, as measured with serial CT scans, and with rate of worsening abnormality of resting state regional cerebral metabolic rates for glucose (rCMRglc) as measured by PET and 18F-Fluorodeoxyglucose. Impairment of complex attention, the first nonmemory function to demonstrate impairment in early DAT, was investigated with cognitive studies which identified three similar impairments of shifting or divided attention. Older Down syndrome adults perform worse on mental abilities tests than do younger subjects.